The present invention is designed to sever from a sheet of thermoplastic material cup-shaped articles which have been integrally formed at uniformly spaced intervals in the sheet. Machines of this general type are known in the prior art, however the present invention is especially directed to the solution of two problems which are almost universally present in previously known machines of this type.
In operations of the type with which the present invention is concerned, the severing operation is normally performed by first seating the cup-shaped article to be severed in a female die with the die surfaces adjacent the die cavity bearing against the back side of the sheet. A reciprocable punch is then advanced into the die cavity by passing through the sheet around the peripheral portion of the article where it is joined to the sheet. In order to feed the sheet to the die means, which is normally stationary in the machine frame, the sheet is passed through a treadle which reciprocates with the punch so that as the punch retracts, the treadle is moved forwardly with respect to the die by a distance at least as great as that by which the article projects axially from the back side of the sheet. When the next succeeding article has cleared the die face, a feed mechanism advances the sheet one step to align the article with the die and the article and sheet are seated against the die by the treadle as the punch moves forwardly in its cutting stroke.
One problem presented by the above arrangement is the fact that the sheet at some point in its path of movement to the treadle normally must pass over and be guided by some structure which is fixedly mounted on the machine frame and must then advance from this fixed point of reference to guides which are fixedly mounted upon and thus continuously movable with the reciprocating treadle. In most prior art machines, no positive guiding action is provided for the sheet during its transit between the fixed point and the moving treadle, and we believe that it is this gap in the guided path of movement of the sheet which leads to feeding problems.
Further, such machines normally must be operated at fairly high cyclic production rates. Particularly where the softer thermoplastic materials are encountered, we believe that, contrary to accepted belief, it is the rapid movement of the punch during the severing operation which causes the punch to extrude or stretch the soft material rather than cleanly severing it from the web.
The present invention is especially directed toward a driving mechanism for the punch where a punch operated at a relatively high cyclic rate is so controlled as to move relatively slowly during that portion of its cycle while it is performing the severing operation, while moving at a more rapid pace during other portions of its cycle.